1. Field of the Invention
The present invention relates generally to devices for producing actinic radiation, and more particularly to devices for producing actinic radiation wherein an electron beam, originating in a vacuum, pierces a thin membrane window to then penetrate into medium present on a non-vacuum environment side of the window.
2. Description of the Prior Art
Actinic radiation is used widely for promoting or inducing chemical reactions in various circumstances such as polymerization, cross-linking, sterilization, grafting etc. Actinic radiation for such purposes can be created by emitting electrons from a cathode ray gun located at one end of a cathode ray tube ("CRT") structure, accelerating the emitted electrons through a vacuum present within the CRT structure, and then directing the electrons onto a very thin anode of a window area Electrons impinging upon the thin anode pass through the window to then produce actinic radiation upon striking atoms and/or molecules in a medium surrounding the CRT structure. Actinic radiation created by such electron beam impingement can either directly or indirectly catalyze chemical reactions which are very difficult to induce by any other means. Because of the nature of the actinic radiation produced by an electron beam impinging into a medium and because the very high power densities obtainable with an electron beam, producing actinic radiation in this way provides a very energetic source of radiation at a cost substantially less than other sources providing comparable performance.
U.S. Pat. No. 4,468,282 entitled "Method of Making an Electron Beam Window," that issued Aug. 28, 1984, on an application filed by one of the applicants herein ("the '282 patent"), describes making a membrane window for such a CRT structure by first depositing a thin film of a refractory material having a low atomic number onto a substrate, and then etching away a portion of the substrate leaving only the thin film. Specifically, the '282 patent discloses depositing a thin film of silicon carbide ("SiC"), boron nitride ("BN"), boron carbide ("B.sub.4 C") silicon nitride ("Si.sub.3 N.sub.4 ") or aluminum carbide ("Al.sub.4 C.sub.3 ") ranging from less than a micron to several microns thick using chemical vapor deposition ("CVD"). The '282 patent further discloses that such a thin film is deposited onto a silicon wafer substrate having a (100) orientation, or onto a suitably selected polycrystalline substrate possibly made from tungsten, molybdenum or silicon. A thin membrane window made in this way from any of the materials listed above is readily permeable to electrons having an energy of 10 to 30 kilo electron volts ("keV"), is inert, pinhole free, has high mechanical strength, and, if deposited under appropriate conditions, has minimal residual stress. A film used for the membrane window, although only a few microns thick, must be vacuum tight and mechanically very strong to withstand atmospheric pressure, while concurrently experiencing thermal stress and heating associated with passage on an electron beam through the film.
A difficulty experienced in fabricating the thin membrane windows disclosed in the '282 patent is that it is difficult to grow a perfect film of most of the suitable materials. Consequently, a significant probability exists that a thin film prepared in accordance with the '282 patent will have approximately one defect square centimeter ("cm.sup.2 ") defect. Such defects weaken the membrane and a single weak point may be sufficient to destroy an electron-beam window, particularly under the high load imposed upon the film due to the difference between atmospheric pressure on one side of the window and vacuum on the other side. Moreover, defects in the thin film may grow or propagate under the combined influences of electron-beam irradiation, heating of the very thin membrane due to impingement upon and passage of the electron beam through the film, and the very high mechanical stress applied by the pressure difference across the window. All the preceding factors cause defects in a membrane to grow which eventually results in catastrophic failure of the film.
Furthermore, several of the thin film materials identified in the '282 patent such as BN and Si.sub.3 N.sub.4 are insulators which is undesirable for various reasons. For example, it has been observed in x-ray lithography that BN and Si.sub.3 N.sub.4 thin films rapidly develop defects upon exposure to electron-beam or x-ray radiation as indicated by the appearance of color centers in the film. Moreover, over time films made from BN and Si.sub.3 N.sub.4 rapidly experience plastic deformation as cumulative electron-beam irradiation increases.
A suitable material for making thin film windows not disclosed or described in the '282 patent is silicon. Silicon has a sufficiently low atomic number so an electron beam will pass through a silicon window, and also has a thermal conductivity that is adequate to permit dissipating energy deposited in the window by passage of the electron beam. Furthermore, a silicon membrane window will not suffer damage by the electron-beam irradiation unless the incident electron-beam energy is 125 keV or greater, an energy level that is far higher than what is usually needed to produce actinic radiation. However, thin film membrane windows made from silicon are useful for this application only if they can be made defect free and of any required thickness.
The methods usually employed to make very thin silicon membranes exploit effects produced by doping pure silicon material. In the most common method for producing thin silicon membranes, silicon is highly doped with boron and then etched with ethylene diamine. However, a thin silicon membrane produced in this way has high internal stress. The stress in such a thin silicon membrane can be reduced if the film is also doped with germanium. However, even with germanium doping the thin silicon membrane exhibits a high dislocation density. Furthermore, the etchant used to make thin silicon membranes in this way, ethylene diamine, is highly carcinogenic and toxic in many other ways.
Alternative methods for making thin silicon membranes rely on electro-chemical etching using an appropriate electrical bias so that etching stops at a junction between p-type and n-type silicon material. Small quantities of thin silicon membranes may be made by electro-chemical etching, but the method is unsuitable for large scale production of membranes. The very heavy doping of the silicon material required to form the junction between p-type and n-type silicon introduces numerous dislocations which reduces the strength of the resulting films. When heated and simultaneously subjected to large mechanical stresses such as those experienced by an electron-beam window, dislocations in the membrane may congregate to form fissures which eventually cause in a catastrophic failure of the membrane.